The present invention relates to a perpendicular magnetic film, a process for producing the same, and a magnetic recording medium having the same. More particularly, the present invention relates to a perpendicular magnetic film which is suitable as a perpendicular magnetic recording material, which is stable against oxidation, which has excellent perpendicular magnetic anisotropy and which is composed of crystals having a small average grain size, a process for producing the same, and a magnetic recording medium having the same.
With recent demand for miniaturization and higher reliability of information processing devices and systems, perpendicular magnetic films have been rapidly developed and put to practical use. A perpendicular magnetic film which is suitable for high-density recording is required to have a large coercive force and a large saturation magnetization in the direction perpendicular to the film surface.
The perpendicular magnetic film is further required to be composed of crystal grains having as small a crystal grain size as possible or to be amorphous, because if the average grain size of the crystals which constitute the perpendicular magnetic thin film is large, the scattering of light takes place on the interfaces of the crystal grains, which causes large noise.
In addition, since the magnetic characteristics of a magnetic recording medium are deteriorated owing to oxidation by the oxygen in the air, the perpendicular magnetic film is required to be stable against oxidation.
Furthermore, when the perpendicular magnetic film is used as a recording layer of a magneto-optical recording medium in which recording and reproduction of information is carried out by using a light beam such as a laser beam, the medium is required to have as high magneto-optical characteristics such as the Faraday rotation angle as possible. With the recent demand for higher-density recording, a tendency is shown that the recorded carrier signals have a high frequency, in other words, that the wavelength of the recorded carrier signals is short. In the case of magneto-optical recording, a recorded bit size is determined by the wavelength of a laser beam, and the shorter the wavelength, the smaller the bit size becomes. A material having a large Faraday rotation angle in a short wavelength region of not more than 600 nm is, therefore, strongly demanded.
Examples of the perpendicular magnetic recording films and processes for producing the same which have conventionally been disclosed are as follows.
(1) A process for producing a cobalt ferrite spinel film, which comprises forming a spinel film by sputtering onto a substrate of a low temperature in an oxidizing air by using an alloy target composed mainly of Co and Fe (Japanese Patent Application Laid-Open (KOKAI) No. 63-47359). In Japanese Patent Application Laid-Open (KOKAI) No. 63-47359, the description of "When a reactive sputtering is carried out in an oxidizing air by using an alloy target composed mainly of Co and Fe, a cobalt.ferrite spinel film represented by Co.sub.x Fe.sub.3-x O.sub.4, (wherein 0.5.ltoreq..times..ltoreq.1.05) is formed on the substrate. In this case, even if the surface temperature is as low as about 200.degree. C., a spinel film having a good crystallizability is obtained." is disclosed.
(2) A magnetic recording medium comprising a substrate, an under layer of a crystalline film having a spinel crystalline structure, and a magnetic recording layer of a spinel ferrite crystalline film, wherein the lattice constant of the under layer is larger than that of the magnetic recording layer (Japanese Patent Application Laid-Open (KOKAI) No. 3-17813). The description of "The under layer is composed of a material represented by the following general formula: EQU AB.sub.2 O.sub.4
wherein A is a metal element of divalent ions and at least one selected from the group consisting of Mg, Mn, Co, Ni, Cu, Zn, Fe, etc., and B is a metal element of trivalent ions and at least one selected from the group consisting of Cr, In, Rh, Sc, Tl, Fe, etc. The magnetic recording layer 3 is composed of the material represented by the following general formula: EQU AB.sub.x Fe.sub.3-x O.sub.4
wherein A and B are the same as in the above-mentioned general formula, and 0.ltoreq..times.&lt;2." is disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 3-17813.
(3) An oriented film of oxide crystals represented by the following general formula: EQU A.sub.x B.sub.3-x O.sub.y
wherein A is at least one selected from the group consisting of Mn, Co, Ni, Cu, Mg, Cr, Zn, Li and Ti; B is Fe or Al;, 0.5.ltoreq..times..ltoreq.2.0 and 2.5.ltoreq.y&lt;4, wherein the ratio (I.sub.111 /I.sub.222) of the reflective peak intensity I.sub.111 of the crystal face (111) and the reflective peak intensity I.sub.222 of the crystal face (222) is less than 0.2 when the X-ray diffraction peak is indexed on the assumption that the film has a spinel crystalline Structure(Japanese Patent Application Laid-Open (KOKAI) No. 3-188604).
(4) A process for producing a cobalt ferrite film comprising the steps of forming a multi-layer metal film by laminating at least two of Co layer and Fe layer on a substrate, and heat-treating the obtained multi-layer film in an air containing oxygen (Japanese Patent Application Laid-Open (KOKAI) No. 4-10509).
There are the following description in Japanese Patent Application Laid-Open (KOKAI) No. 4-10509.
"The total thickness of the Co layer and the Fe layer in the multi-layer metal film is not more than 100 .ANG.. This is because if the total thickness of the Co layer and Fe layer exceeds 100 .ANG., it is difficult to produce a cobalt ferrite film having a large Kerr rotation angle."
"Seven kinds of multilayers were formed on a glass substrate 3 (Coning 7059, produced by CONING) by sputtering in Ar while using a single Co target and a single Fe target under the following condition, as shown in FIG. 1.
Sputtering condition:
Total sputtering pressure: 2 mtorr PA1 Sputtering current: 0.2 A PA1 Substrate temperature: room temperature PA1 (wherein A is a transition metal such as Co, Ni, Mn, etc., and x is not more than 1), and PA1 (wherein B is a transition metal such as Fe, Ni, Mn, etc., and y is not more than 1), PA1 (wherein A is a transition metal such as Co, Ni, Mn, etc., and x is not more than 1), PA1 (wherein A' is a transition metal such as Ni, Mn, etc., a is not more than 1, and z is more than 0 and less than 1 and changes continuously between 0 and 1), and PA1 (wherein B is a transition metal such as Fe, Ni, Mn, etc., and y is not more than 1), PA1 (wherein A is a transition metal such as Co, Ni, Mn, etc., and x is not more than 1), and PA1 (wherein B is a transition metal such as Fe, Ni, Mn, etc., and y is not more than 1), PA1 (wherein A is a transition metal such as Co, Ni, Mn, etc., and x is not more than 1), PA1 (wherein A' is a transition metal such as Ni, Mn, etc., a is not more than 1, and z is more than 0 and less than 1 and changes continuously between 0 and 1), and PA1 (wherein B is a transition metal such as Fe, Ni, Mn, etc., and y is not more than 1),
In each of the multilayers, the thickness ratio of a Co layer 1 and an Fe layer 2 was 1:2, and the total film thickness was constantly 2000 .ANG.. These multilayers obtained were 1 layer of (Co/Fe=660 .ANG./1340 .ANG.), 2 layers of (Co/Fe=330 .ANG./670 .ANG.), 4 layers of (Co/Fe=165 .ANG./335 .ANG.), 8 layers of (Co/Fe=82.5 .ANG./167.5 .ANG.), 10 layers of (Co/Fe=66 .ANG./134 .ANG.), 20 layers of (Co/Fe=33 .ANG./67 .ANG.), and 40 layers of (Co/Fe=17 .ANG./33 .ANG.). Each of the multilayers was heat-treated in an electric oven in the air in the heat treatment pattern shown in FIG. 3. More specifically, the film was heated at a high raising rate until 300.degree. C., and then heated at a raising rate of 100.degree. C./hr until 500.degree. C. The film was heated at 500.degree. C. for 2 hours, and was then gradually cooled. Thus, a cobalt ferrite film was formed on the glass substrate"
Namely, the cobalt ferrite films disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 4-10509 are a multilayer (in-plane magnetic film) composed of a cobalt layer and a ferrite layer and a film (perpendicular magnetic film) composed of a cobalt.ferrite oxide. Each of the multi-layer metal films is heat-treated at a temperature of not lower than 500.degree. C. for not less than 2 hours, thereby obtaining a cobalt.ferrite oxide film.
(5) An amorphous alloy film such as a Gd-Co film and a Tb--Fe film, composed of a rare earth metal and a transition metal (Japanese Patent Application Laid-Open (KOKAI) No. 51-119999) and a magneto-plumbite oxide thin film such as a barium ferrite film (Japanese Patent Application Laid-Open (KOKAI) No. 62-267949).
While the above-mentioned amorphous alloy film is advantageous in that the noise produced is small because there is no grain boundary, it is disadvantageous in that the magneto-optical characteristic (Kerr rotation angle) is lowered to less than 0.3 deg. which is necessary in practical use, when a laser beam having a short wavelength is used, and in that the film is unstable against oxidation, that is, it is liable to be oxidized. Further, the above-mentioned oxide thin film is advantageous in that it is stable against oxidation, but it is disadvantageous in that since the grain size is large, large noise is produced. The crystallinity of a film has a close relationship to various characteristics such as perpendicular magnetic anisotropy and magneto-optical characteristics. In order to improve various characteristics of a film, a high-temperature heat-treating process is essential in the formation of the film. For example, the substrate is heated to not lower than 500.degree. C., or after forming a film on the substrate at a low temperature such as not higher than 400.degree. C. the obtained film is heat-treated at a high temperature such as not lower than 500.degree. C. As a result, crystal grains necessarily grow. When the average grain size becomes large, light greatly scatters, resulting in large noise at the time of reading the information recorded in the magneto-optical recording.
In order to heat the substrate to not lower than 500.degree. C., the substrate itself is required to have a high heat resistance. Nevertheless, the heat resistance of a material which is now generally used for the substrate of a perpendicular magnetic recording medium such as polycarbonate and epoxy resin is insufficient, so that the material for the substrate is restricted, which is disadvantageous from the point of view of industry and economy.
None of the above-mentioned films sufficiently satisfy all requirements such as excellent stability against oxidation, excellent perpendicular magnetic anisotropy and crystals of a small average grain size.
Accordingly, a perpendicular magnetic film which has excellent perpendicular magnetic anisotropy at a temperature of not higher than 500.degree. C. and which is composed of an oxide having a small average grain size, is now in strong demand.
As a result of various studies undertaken by the present inventor, it has been found that (1) a perpendicular magnetic film obtained by forming a unit (A)/(B) or (B)/(A) composed of a spinel type crystal layer (A) containing Fe as the main ingredient and a spinel type crystal layer (B) containing Co as the main ingredient and disposed in the order of (A) and (B) or (B) and (A), and laminating at least two such units, or (2) a perpendicular magnetic film obtained by forming a unit (A')/(C)/(B')/(C) or (B')/(C)/(A')/(C) composed of a spinel type crystal layer (A') containing Fe as the main ingredient, a spinel type crystal layer (B') containing Co as the main ingredient, and a spinel type crystal layer (C) containing a mixture of Fe and Co as the main ingredient and disposed in the order of (A'), (C), (B') and (C) or (B'), (C), (A') and (C), and laminating at least two such units, is stable against oxidation (is excellent in an oxidation resistance), has excellent perpendicular magnetic anisotropy and has a small average grain size, and as a result, it is suitable as a magneto-optical recording material. On the basis of this finding, the present invention has been achieved.